Expression of desiccation-induced and lipoxygenase genes during the transition from the maturation to the germination phases in soybean somatic embryos

Transitions between developmental programs during plant embryogenesis are usually characterized by differential gene expression. Soybean (Glycine max (L.) Merr.) somatic embryos from embryogenic suspension cultures germinate poorly on a germination medium even after being grown on medium containing abscisic acid until developmental changes are induced by desiccation. The objectives of this study were to investigate: (i) the relationship between water loss and germination of somatic embryos, (ii) changes in desiccation-induced polypeptides, and (iii) the expression of maturation-and germination-associated genes during the transition from maturation to germination of soybean somatic embryos. The results revealed that partial drying of somatic embryos at 76% relative humidity for 96 h resulted in more than 90% germination. Germination frequency was correlated with the percent fresh weight loss of the somatic embryos. Polypeptides induced by desiccation were classified into three groups based on the tissues in which they were expressed and the inducing agents to which they responded. Desiccation-induced polypeptides decreased or became undetectable upon imbibition. The expression of a maturation-associated gene, Mat1, was induced in both cotyledons and hypocotyl/radicle tissues of somatic embryos after 72 h desiccation. The transcript for this gene increased in concert with water loss from the embryos. During maturation of somatic embryos, the expression of embryonic lipoxygenase genes was turned on. Desiccation alone did not directly induce expression of germination-associated lipoxygenase genes SC514 and LOXB2 in somatic embryos. However, upon rehydration for 24 h and prior to any visible developmental change in embryos on the germination medium, the lipoxygenase transcripts and proteins became detectable in germinable somatic embryos but not in non-germinable somatic embryos. These data suggest that for somatic embryos which do not complete their normal developmental processes, desiccation is required for the transition from a maturation to a germination program.